So, on to my next attempt at fixing what seems like a never ending list of problems. As soon as I solve one, another pops up.

I've got a 87 300SDL. If I get another one of these cars, I'm going to go older and avoid anything "auto."

When i unlock my car, all four door locks go up. When I lock it, only three go down. The rear driver's side lock wont drop. I have to push it down in order to lock it (goes down pretty easy).

Since it goes up (an all the others work fine), I'm guessing it's not a vacuum issue. Would I be wrong in this assumption.

Any ideas on where to start?

If you're familiar with the 123, the 126 has a slightly different central locking system. The system runs off a pump beside the spare tire which provides vacuum to lock and pressure to unlock. It's an odd failure that the actuator responds to pressure but not vacuum but that's where I'd start. It could be a linkage problem between the actuator and latch.

Sixto
87 300D

I'm not familiar with the 123. The 300SDL is my first.

When you say linkage, does that mean mechanical, or vacuum? Would there be a hose I would look for inside the door, or back by the spare?

If it works in one direction, it's probably not a problem with the vacuum/pressure line. By linkage I mean the rods and bell cranks that go between the actuator and latch. Now that I think about it, does the door lock even if the button stays up?

Sixto
87 300D

I'd suspect it's a torn diaphragm in one of the vacuum actuators. When the leak is small the pump's pressure output is enough to overcome it, and the door will unlock. But because of how the diaphragm flexes when locking, the vacuum leak is too great and the door won't lock.

The leak is probably in the door that doesn't work properly. Easy enough to test with a hand pump... or just start the locking cycle and listen for a hissing sound. You'll need to replace the actuator, repairs don't last long (ask me how I know...).

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

No, if it's up it is unlocked. I thought that too. The only way to lock it is the press it down.

Holy Smokes! At $92 I think I'll just keep pressing it down. maybe I'll find one in parts car at some point.

Thanks!

On the W126, that is a single line system that provides both pressure and vacuum from the pump. Easy enough to check by pulling the cover off the "B" pillar and using a mightyvac pump to see if the line holds vacuum. It may be something as simple as a flex line at the actuator.

However, I'm with Sixto on this one. If there is virtually no tension on the actuator when switched to lock, I would inspect the actuator for something mechanical that may have come loose.

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Sam

84 300SD 350K+ miles ( Blue Belle )

WOW!!! $92 is a bit pricy for the actuator.
I would start scouring the Junk Yards or PNP type yards for an actuator. There is a lot of good parts in them thare yards.


Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

Guys $92 does not sound right. I have an '81 300SD and I replaced the actuator for about $25. This was maybe two or three years ago. Unless the SDL part is just a lot more costly, it should cost about the same. Did you try Pelican Parts?

That's where I checked for the $92 amount. But I found one used elsewhere for much cheaper. I'm gonna give it a try.

Priced them lately? The SD parts are the same. Some of these parts may need to be stock-piled when they are cheaper. Charlie may have the right idea....those salvage spares may be increasing in value at a rapid pace.

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Sam

84 300SD 350K+ miles ( Blue Belle )